1. Field of the Invention
This invention is related to the field of chemistry. More particularly this invention is related to solid fuel propellants and gas generators, more particularly to urethane binders based upon curing of Glycidyl-Azide Polymer (GAP)/isocyanate mixtures, processes for their cure, and catalysts suitable for use therein. This invention is also related to polyurethane casting resins in general, catalysts for their cure, processes for their use, and cured articles formed thereby.
2. Description of the Prior Art
Composite solid propellants normally comprise a rubbery phase which is intimately admixed with and serves as a binder for such additional solid ingredients as ammonium perchlorate or other oxidizers, finely divided aluminum, or other metallic fuel, and to a minor extent such other additives as, for example, iron oxide. The polyurethane binder system is known to impart superior physical and mechanical properties to the propellant. U.S. Pat. No. 4,184,031 to Graham et al., the disclosure of which is incorporated by reference, is illustrative.
Gun propellant binders of nitrocellulose plasticized with GAP are known in the prior art such as the copolymerization with nitrocellulose illustrated in U.S. Pat. No. 4,288,262 to Flanagan et al., the disclosure of which is incorporated by reference. Heretofore, reaction mixtures of glycerol initiated GAP and isocyanates using known effective catalysts in the presence of some commonly used propellant ingredients had a tendency to gas such that the cured propellants produced were not useable. Even a relatively small percentage of gas bubbles, which produce voids in the cured solid propellant grain, can result in uncontrolled and unpredictable burn rates which can result in explosion of the rocket motor or gas generator. The presence of these voids also undesirably reduces the density and thus the available energy from a given sized propellant grain.
Various catalysts known to promote urethane linkage have been tried but found unsatisfactory. Most catalysts, triphenyl bismuth (TPB) as well as several tin containing catalysts caused unacceptable gassing. The iron acetylacetonate was found effective, however, it caused thermal degradation of the azide polymers and was thus unusable. The TPB catalyst was further noted as having low activity in curing GAP/isocynate mixtures.
Due to the energetic properties of GAP, its use in a propellant as a binder would be highly desired over non-energetic prior art binders since a higher specific impulse performance is attained. This is particularly desirable in volume limited missile applications where range of existing missiles could be extended.